Hybrid electric vehicles offer reduced fuel consumption and emissions by utilizing one or more electric motor/generators, an internal combustion engine, and an onboard controller programmed to vary use of each of the engine and motor/generator(s) during different driving conditions to achieve peak efficiency.
Powertrains with an electrically-variable transmission typically have an input member, an output member, and two electric motor/generators connected to different members of planetary gear sets. Engagement of torque-transmitting mechanisms may allow one or more electrically-variable modes of operation, fixed speed ratio modes, and an electric-only (battery-powered) mode. Electrically-variable transmissions may improve vehicle fuel economy in a variety of ways, primarily by using one or both of the motor/generators for vehicle braking and using the regenerated energy to power the vehicle electrically, with the engine off. The engine may be turned off at idle, during periods of deceleration and braking, and during periods of low speed or light load operation to eliminate inefficient engine operation and engine friction losses. Braking energy captured via regenerative braking (or electrical energy generated during periods when the engine is operating) is utilized during these engine-off periods. Transient demand for engine torque or power is supplemented by the motor/generators during operation in engine-on modes, allowing for a smaller engine without reducing vehicle performance. Additionally, the electrically-variable modes may allow the engine to be operated at or near the optimal efficiency point for a given power demand. For example, an EVT may be configured in an input-split mode, in which one motor-generator is connected through gearing only to the output, and functions as a motor at low speed; a second motor generator is connected through gearing to both the input and the output, and functions as a generator at low speeds. An EVT may be configured in a compound-split mode, wherein both the first and second motor-generator are connected through gearing to both the input and output.